1. Field of the Invention
The present invention relates to a point to multipoint device for use in a wireless network to provide wireless communication with a plurality of telecommunication units, to a method of operating such a device, and to a wireless network comprising a plurality of such point to multipoint devices.
2. Description of the Prior Art
Point to multipoint devices within a wireless network may take a variety of forms. For example, such a point to multipoint device may take the form of a relay node or repeater used to propagate data within the wireless network. Such relay nodes or repeaters typically amplify and forward or decode and forward received signals within the wireless network. Another example of a point to multipoint device would be a base station associated with a cell of the wireless network for communicating over wireless links with a number of subscriber stations and/or relay nodes in the cell.
A wireless network infrastructure typically subdivides a geographic area into mutually disjoint regions called cells. Associated with each cell are one or more base stations (BSs) that communicate via radio signals with a number of subscriber stations (SSs) located within the same cell. The transmission path from the BS to the SS is known as the forward link or downlink communication path, whilst the transmission path from the SS to the BS is known as the reverse link or uplink communication path.
In one implementation, the BS may be connected to a telephone network and exists to relay messages from SSs in the cell controlled by the BS to the telephone network, and vice versa. By this approach, an item of telecommunications equipment connected to an SS may make an outgoing call to the telephone network, and may receive incoming calls from the telephone network.
However, such a wireless telecommunications system is not restricted to use with telephone signals, but could instead, or additionally, handle any other appropriate type of telecommunications signal, such as video signals, or data signals such as those used for transmitting data over the Internet and in order to support recent technology such as Broadband and video-on-demand technologies.
Within such a wireless network, a measure of the received signal quality is the Signal to Interference and Noise Ratio (SINR). For a given SINR, the receiver (for example the receiver provided at an SS) can request a suitable Modulation and Coding Scheme (MCS) that will maximise the data rate and at the same time ensure an acceptable Quality of Service (QoS). The Frame Error Rate (FER), i.e. the percentage of blocks of data that are received in error, is frequently used as a measure of the QoS. If a block of data is incorrectly decoded, then the receiver will inform the transmitter (for example the transmitter at the BS when considering the receiver at the SS) to resend the data. Whilst such a scheme is necessary to maintain an acceptable QoS, data repetition has the drawback of reducing the overall system throughput.
Cell sectorisation is a well-known technique for increasing the system capacity, system capacity being a measure of the ability of a network to serve and sustain simultaneous users. In cell sectorised layouts, the area within a cell is, under ideal situations, sub-divided into a number of non-overlapping regions called sectors. The sectors within the same cell are served by the same BS, or by different BSs (one per sector). In such sectorised layouts, the point to multipoint device may be considered to be the entire base station, or the sector specific logic, whether that be provided as a physically separate base station or as a part of a base station covering the entire cell. Sectorisation is generally implemented by employing highly directional antennas that concentrate the radiated energy within a sector. FIG. 1 shows a cellular network consisting of seven cells, with each cell comprising three sectors. Hence, by way of example, the cell 10 illustrated in FIG. 1 is served by a base station 20 which can provide separate beams to cover the three sectors 30, 40, 50 provided within the cell 10.
Typically, a BS may need to communicate simultaneously with multiple SSs within a sector or a cell. Typically, such simultaneous communication can be achieved by defining multiple communication channels that can be arranged to utilise the radio resource of the wireless network. For example, in a “Time Division Multiple Access” (TDMA) system, a particular frequency channel can be partitioned in the time domain, such that a number of different signals can be transmitted in different time slots, the time slots forming multiple communication channels utilising the particular frequency channel. As another example, in a “Frequency Division Multiple Access” (FDMA) system, a band of frequencies may be partitioned to form a number of communication channels of particular frequencies, thereby enabling multiple signals to be transmitted over the radio resource. In a combined TDMA/FDMA system, such as used in WiMAX systems, a combination of time/frequency slot is used to define separate communication channels. WiMAX systems are based on the IEEE 802.16 standards that provide high-throughput broadband connections over relatively long distances.
As another example of a mechanism that can be used to establish multiple communication channels within a radio resource, in a “Code Division Multiple Access” (CDMA) system, signals may be transmitted over the radio resource on a particular frequency channel, and this frequency channel may be partitioned by applying different orthogonal codes to signals to be transmitted on that frequency channel. Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal communication channel utilising a particular frequency channel.
The total number of resources (i.e. channels) in a wireless network is limited. In order to increase the system capacity it may be necessary to use the same channel in different cells and/or sectors. This is known as channel re-use. The cells or sectors that use the same set of channels are known as co-channel cells or sectors, and the interference generated as a result is referred to as Co-Channel Interference (CCI). CCI degrades the quality of the received signal and thus CCI impacts negatively on the system throughput. Considering again FIG. 1, it will be noted that there are crossover regions between adjacent sectors in FIG. 1. CCI in these locations will be high, and can be avoided by using different channels on overlapping sectors.
Another way to mitigate the CCI is to use antenna arrays at the BS, such antenna arrays being described for example in Chapter 3 of the publication “Smart Antennas, Adaptive Arrays, Algorithms, and Wireless Position Location”, edited by Dr T S Rappaport, IEEE, NJ 1998, that chapter providing an introduction to smart antennas and spatial processing. An advanced (also referred to in the art as smart) antenna array consists of two or more closely spaced antennas, and in combination with a beamforming network, narrow beams with increased signal strength can be formed in the direction of the desired SS. Exploiting the spatial separation between users, the advanced antenna array can also reduce the interference to other SSs. The overall benefits of antenna arrays are increased range and improved signal strength (due to the antenna array gain), along with increased system capacity due to the efficient utilisation of spectral resources, i.e. reduced CCI.
One known type of smart antenna array is referred to as a fixed multi-beam antenna array system, where a finite number of fixed beams with predefined beam patterns and fixed pointing directions are employed. Another alternative type of smart antenna is the steered beam, or fully adaptive, antenna system. Unlike the fixed multi-beam systems, a steered beam system can radiate its energy in any direction, and in some cases can ensure little or no interference (nulling) in certain other directions.
Considering the uplink communication from a SS (or more generically a telecommunications unit) to a BS (or more generically a point to multipoint device), each BS can define a sequence of variable duration communication channels to be used for the uplink communication, and particular SSs within the cell or sector will then be allocated one of those communication channels as and when required to enable communication to take place on the uplink between that SS and the associated BS. Typically the communication is broken down into separate frames, and the sequence of variable duration communication channels can be defined for each frame. One example of a communication structure that can be used is shown in FIG. 2, which illustrates a Time Division Duplex (TDD) communication structure, where uplink and downlink transmissions occur at different times but share the same frequency. An example of a system that may use such a TDD communication structure is a WiMAX system, where the TDD mode is implemented by subdividing each frame into a downlink subframe and an uplink subframe. Accordingly, as shown in FIG. 2, the communication takes place between a BS and a plurality of associated SSs via a series of frames 100. Considering for example the frame n 105, this consists of a downlink (DL) subframe 110 and an uplink (UL) subframe 115. Considering the UL subframe 115, this is subdivided into one or more variable duration communication channels, referred to in FIG. 2 as bursts 120, 125, 130. Each burst is preceded by a training sequence TS, such training sequences being used for uplink channel estimation, i.e. to allow the receiving BS to estimate the amount of distortion in that channel.
Due to the fact that the SS allocated to any particular uplink communication channel can vary between frames, and given that the length of each communication channel can vary from frame to frame, this can give rise to varying degrees of CCI within the uplink subframe. This problem is illustrated farther with reference to FIGS. 3 and 4. As shown in FIG. 4, an example configuration has two SSs, namely SS1 305 and SS2 310, associated with a first base station, BS1 300, whilst a further SS, namely SS3 320, is associated with a second BS, namely BS2 315. During the time interval T1, shown in FIG. 3, the signal 200 from SS1 305 is received at BS1 300 and BS2 315, as shown in the left-hand half of FIG. 4. The signal from SS1 acts as a co-channel interferer to the desired signal 220 from SS3 320 at BS2 315. Similarly, the signal from SS3 320 acts as a co-channel interferer to the desired signal from SS1 305 at BS1 300. In this example, it is assumed that the SSs SS1, SS2 and SS3 all communicate on the same frequency channel, and accordingly the signals from SS1 and SS3 cause CCI during the time interval T1. During the time interval T2, the signal 220 from SS3 320 will continue to act as a co-channel interferer at BS1 300, but the signal 210 from SS2 310 is the new co-chamnel interferer at BS2 315. Hence, as shown in FIG. 4, it can be seen that in addition to the desired signals 325, there are also interfering signals 330 that contribute to CCI.
The CCI can be mitigated using smart antennas. If a fixed multiple-beam antenna array system is used, then BS1 300 can select a fixed beam to be used in the uplink to enhance the signal quality of the signals from SS8 305 and SS2 310, whilst suppressing the interference arising from the signal issued by SS3 320. Likewise, a fixed multi-beam system in BS2 315 would be capable of suppressing, to some extent, the interference resulting from the signals issued by SS8 305 and SS2 310.
If a steered beam antenna system is used instead of a fixed multi-beam antenna system, then the training sequences issued at the beginning of each communication channel may be used by the associated BS to estimate the channel impulse responses from the SSs to the BSs and/or the array response at the BSs. Broadband or narrow band beamformers can be designed to collect the energy from the desired SSs. In some embodiments, the broadband or narrow band beamformers can be designed to not only collect the energy from the desired SSs, but also optionally place nulls in the direction of any SS introducing CCI. Considering again the example of FIG. 3, it can be seen that BS1 300 can based on the training sequence 205 produce a first steered beam to use during time period T1, and then based on the training sequence 215 produce a different steered beam to use during the time period T2, and in both cases these steered beams may be arranged to place a null in the direction of SS3 320. However, in contrast, BS2 315 will use the training sequence 225 to produce a steered beam which is then used for the entire duration of the communication channel 220 used by SS3 320 to communicate uplink data to BS2 315. Whilst this may place a null in the direction of SS1 305, since that was the source of CCI at the time the beam was calculated, it will not be able to take account of the new source of CCI, namely SS2 310 that is introduced part way through transmission on that communication channel 220, and accordingly this will give rise to an elevated level of CCI during period T2. Indeed, it may well be the case that when using a steered beam antenna system, the CCI from SS2 will not only not be sufficiently suppressed, but may in some instances actually be amplified.
Accordingly, it would be desirable to provide a technique which enabled a further reduction in CCI within a wireless network.
US 2004/0235527 discusses techniques for CCI suppression on the downlink transmission path. The techniques do not address the varying degrees of CCI within the uplink subframe. As discussed earlier, the CCI in the uplink path is due to the presence of the variable duration communication channels when the SSs communicate to the BS. Since a certain BS does not know the duration of the communication channels used by the SSs in other cells, the BS cannot suppress the uplink interference caused by the intercell interfering SSs. WO 2004/059879 suggests using beamforming at a mobile device for reception, and then applying the same beamforming weights for transmission on the uplink path. The technique is applicable to TD-SCDMA (Time Division Synchronous Code Division Multiple Access) mobiles and not for BSs that are subject to uplink CCI originating from intercell variable duration communication channels.